This doesn't mean there could not be particles of much higher energies, but they would be extremely scarce and could not be found without a far more sensitive detector.

It would have been fun to find a violation of the rule, but to see it actually working is "a pretty good feeling," he said.

So what did Fly's Eye see in 1991?

"I guess we have to believe at this point it was an artifact. We've not been able to replicate it, and our sensitivity has increased well beyond where we should have seen a large number of those things," he said.

Sokolsky speculated the earlier Utah report may have involved a low-energy cosmic ray hitting a layer of haze in the atmosphere, creating what looked like a high-energy cascade of particles. That would seem to "fake a high-level event," he said.

Charles Jui (pronounced Ray), professor of physics and, like Sokolsky, a co-author of the paper, said searching for evidence for or against the cutoff was the goal of HiRes. Data collected before were not abundant enough to reach a definite conclusion, but now enough observations have been made.

"People had hoped that maybe we would find some new physics," he said. The new study verifies standard physics and shows that the cutoff is real.

The finding also means that there are few sources of high-level cosmic rays in this vicinity in space, Jui said. "If there are local sources, we would probably see more ... at the highest energies."

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